Oncogene studies on spontaneous and chemically induced rodent neoplasms started 7 years ago in collaboration with molecular biologists at NIEHS and are continuing. Emphasis has been on the patterns of oncogene activation in chemically induced versus spontaneous lung and liver tumors in rats and mice using samples from National Toxicology Program bioassays as well as from rodent studies designed to investigate strain susceptibility to genotoxic and nongenotoxic chemical carcinogens. Important information useful in interpreting bioassay results and in rick assessment has been generated from these studies and underscores the need to continue similar investigations. Current work has been expanded to investigate the role of suppressor genes in rodent carcinogenesis to identify susceptibility factors that impact on carcinogenesis.